I already had some success on Reddit, where I created interesting map charts with it (like religiosity by US state, most popular browser by country,etc.) and shared the link to the site in the comment. This got me many users that would either come directly from the Reddit post or see another map somewhere and google my site.
The big spike came last May, when I shared it on /r/InternetIsBeautiful (https://www.reddit.com/r/InternetIsBeautiful/comments/4jcadt...) , which is a specific subreddit for internet tools and single-purpose website, and it propelled to the front page.
The competition in this niche is either stale (old sites that don't have a modern interface and/or many options) or premium services (whole suites of applications that cover a range of options), so it is currently the #1 Google Search result for queries like 'create my own map', 'make a world map', 'custom USA map' etc.
It is completely free to use and my only source of income are Adsense ads on the homepage and some sparse donations. Since May, when the site was featured on the front page of Reddit, my audience has increased with a steady rate, along with my ad revenue (almost $500 last month).
I don't really do anything maintenance-wise, except for answering questions from my users. I work on it from time to time, mainly adding new features or new country/regions maps, and I still enjoy it, as it was my first real programming project (I actually submitted it as a final project for EdX's CS50x course!).